[16.0][FIX] account_payment_line - wrong compute

[kittiu]

Note: This module extend this #666

Before this fix, things (sign + or -) seem to go wrong on Supplier Payment side, especially when use together with write off.

For example,

1. When click Propose Payment Distribution, the allocation amount is (+), while I think this should be (-) as other columns (otherwise it is very confusing)

2. Then I click on Full Paid the writeoff amount become -200

3. Or I choose to allocate only partial 90 out of 100, and tick Full Paid again, I would expect to write off = -10, but it is -190

4. Or I redo again, 1) Amount 190, 2) system allocate, tick full paid 3) click Confirm

I also wonder why there are so much bugs here. But I think the previous author focus on testing for the Customer Payment side, which seem to be OK.

So, this PR is trying to make it works on both side.

[kittiu]

@ChrisOForgeFlow as maintainer of this module, can you help check if I am not misunderstanding your design intention and the fix is valid.

Thanks!

[ChrisOForgeFlow]

@ChrisOForgeFlow as maintainer of this module, can you help check if I am not misunderstanding your design intention and the fix is valid.

Thanks!

Just like you say, my principal focus was on customer side, I added some supplier test, but I think is good to add your fixes in main version, and maybe we can add a backport

[github-actions]

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