fix: Write Off not working when paid_amount is greater than grand_total

[devdiogenes]

Write-Off amount is currently not working when the paid amount is greater than the grand_total.

To simulate, lets suppose that we're selling an item that costs $ 8.50, then we only give back $ 1.00 to the customer, and the customer says that we don't need to give back the $ 0.50 amount.

Item value: $ 8.50
Change amount: $ 1.00
Write-off: $ -0.50

This is how the system behaves before apply changes:

This commit apply all changes necessary for make it works properly. This is how the system behaves after applying changes:

I also changed POS Invoice Merge Log, cause after we make the POS Closing Entry, the write-off amounts are not summed in the consolidated Sales Invoice.