How to compare two text files and and change the sign of the data in powershell?

I have two text files which different columns and i want to compare two text files data ..and if 1 column data in 1st text file matches with another text file column (P-O-V) data, all the signs of the 2nd text file COLUMNS (JAN TO DEC) data have to change (+ TO - OR - TO +).

sample1.txt

100000 RS
200000 RS
300000 RS

sample2.txt

Sc,January,February,March,April,May,June,July,August,September,October,November,December,P-o-v,D L C N 


P2,3.24,5.67,-2.31,2.82,-2.93,-5.08,2.40,-2.90,2.90,-2.90,2.90,2.90,"001,100100,99001,P000000,U,D-0",PS

P2,3.24,5.67,2.31,2.82,2.93,5.08,2.40,2.90,2.90,2.90,2.90,2.90,"001,100000,99001,P000000,U,D-0",PS

P2,-3.24,5.67,-2.31,2.82,-2.93,5.08,2.40,-2.90,2.90,2.90,-2.90,2.90,"001,200000,99001,P000000,U,D-0",PS

P2,3.24,-5.67,2.31,-2.82,2.93,5.08,-2.40,2.90,2.90,-2.90,2.90,2.90,"001,300000,99001,P000000,U,D-0",PS

P2,-3.24,5.67,2.31,-2.82,2.93,5.08,-2.40,2.90,-2.90,2.90,-2.90,2.90,"001,300000,99001,P000000,U,D-0",PS

i want output file as output.txt

Sc,January,February,March,April,May,June,July,August,September,October,November,December,P-o-v,D L C N 
P2,3.24,5.67,-2.31,2.82,-2.93,-5.08,2.40,-2.90,2.90,-2.90,2.90,2.90,"001,100100,99001,P000000,U,D-0",PS

P2,-3.24,-5.67,-2.31,-2.82,-2.93,-5.08,-2.40,-2.90,-2.90,-2.90,-2.90,-2.90,"001,100000,99001,P000000,U,D-0",PS

P2,3.24,-5.67,2.31,-2.82,2.93,-5.08,-2.40,2.90,-2.90,-2.90,2.90,-2.90,"001,200000,99001,P000000,U,D-0",PS

P2,-3.24,5.67,-2.31,2.82,-2.93,-5.08,2.40,-2.90,-2.90,2.90,-2.90,-2.90,"001,300000,99001,P000000,U,D-0",PS

P2,3.24,-5.67,-2.31,2.82,-2.93,-5.08,2.40,-2.90,2.90,-2.90,2.90,-2.90,"001,300000,99001,P000000,U,D-0",PS

i have tried this code..which is not working

$sample1 = Get-Content -path "sample1.txt"
$Sample2 = import-csv -Path "sample2.txt" -Header "Sc","January","February","March","April","May","June","July","August","September","October","November","December","P-o-v","D L C N"

  if ($sample2 -contains $sample1) {

 foreach($row in $sample2)
{
$row.January = "{0:N2}" -f (-1 * [float]$row.January)
$row.February = "{0:N2}" -f (-1 * [float]$row.February)
$row.March = "{0:N2}" -f (-1 * [float]$row.March)
$row.April = "{0:N2}" -f (-1 * [float]$row.April)
$row.May = "{0:N2}" -f (-1 * [float]$row.May)
$row.June = "{0:N2}" -f (-1 * [float]$row.June)
$row.July = "{0:N2}" -f (-1 * [float]$row.July)
$row.August = "{0:N2}" -f (-1 * [float]$row.August)
$row.September = "{0:N2}" -f (-1 * [float]$row.September)
$row.October = "{0:N2}" -f (-1 * [float]$row.October)
$row.November = "{0:N2}" -f (-1 * [float]$row.November)
$row.December = "{0:N2}" -f (-1 * [float]$row.December)
}

$sample2 | Export-CSV -UseQuotes Never -path "output.txt"

Write-Host "File  was modified" -foregroundcolor green
 } else { 
  Write-Host "File  was not modified" -ForegroundColor  red 
 }

If I understand your question correctly, you want to flip the signs on all values in columns "January".."December" if in that row the value from sample1.txt (first column only) can be found as the second value in the P-O-V column of file 2.
That 'P-O-V' column itself is a comma separated set of values.

Try

# import the csv data from file 2  (as per your example, it has headers)
$data = Import-Csv -Path 'D:\Test\sample2.txt'
# get the text from file 1 and loop over the lines
Get-Content -path "D:\Test\sample1.txt" | ForEach-Object {
    $valueToMatch = $_ -replace '\D+'  # just the number
    # try and find rows in the csv data with a matching value
    $matchData = $data | Where-Object { ($_.'P-o-v' -split ',')[1] -eq $valueToMatch }
    if (@($matchData).Count) {
        foreach ($row in $matchData) {
            # flip the signs in the month columns
            "January","February","March","April","May","June","July",
            "August","September","October","November","December" | ForEach-Object {
                $row.$_ = '{0:F2}' -f (-[double]$row.$_)
            }
        }
    }
}

# export the updated data. I'm setting parameter `UseQuotes` to `AsNeeded`, because column `P-o-v` is a 
# comma delimited string and should not be split apart into several fields
$data | Export-Csv -Path 'D:\Test\output.csv' -UseQuotes AsNeeded -NoTypeInformation

<!-- language-all: sh -->

For better performance, I suggest loading the numbers to compare the files by into a hash set first:

# Import the TSV file and load the values from the first column into a hash set.
$hashset = [Collections.Generic.HashSet[string]] (
  Import-Csv -Delimiter "`t" sample1.txt -Header Number, Unused
).Number

# Import the CSV file and, in ever row that contains a hash-set entry
# in the 2nd ","-separated field of the "p-o-v" column, invert all column
# values that contain a floating-point number.
Import-Csv sample2.txt |
  ForEach-Object {
    if ($hashset.Contains(($_.'p-o-v' -split ',')[1])) {
      # Invert the signs in all relevant columns.
      foreach ($prop in $_.psobject.Properties) {
        if ($prop.Value -as [double]) {
          $prop.Value = if ($prop.Value.StartsWith('-')) { $prop.Value.Substring(1) } else { '-' + $prop.Value }
        }
      }
    }
    $_ # Output the (potentially modified) row object.
  } | ConvertTo-Csv  # replace with Export-Csv ... as needed in your real code.