[FIX] account: fix payment_difference computation
[FIX] account: fix payment_difference computation

Bug introduced by https://github.com/odoo/odoo/commit/cdca5d4751a979f6b694f02d88160843f81a77f5

There is two problems:
- The payment difference is the difference between what you need to pay and what you pay.
=> The compute does the opposite.
- When the payment type is outbound, the payment amount is never negative.
=> Need to inverse the payment amount in the compute in this case.

E.G.

Suppose A a customer invoice of 25 and B a vendor bill of 50:

Payment on A will compute:
payment_type = 'inbound'
amount = 25

If you pay 20, the payment_difference computed was 20 - 25 = -5
It should be 25 - 20 = 5 instead.

Payment on B will compute:
payment_type = 'outbound'
amount = 50 (not -50!)

If you pay 40, the payment difference computed was 40 - (-50) = 10
It should be -50 - (-40) = -10 instead.

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