fix: set correct unallocated amount in Payment Entry (backport #43958)

[mergify]

Closes #43844

Squash and Merge

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This is an automatic backport of pull request #43958 done by Mergify.

[mergify]

Cherry-pick of 7cc111f has failed:

On branch mergify/bp/version-15-hotfix/pr-43958
Your branch is up to date with 'origin/version-15-hotfix'.

You are currently cherry-picking commit 7cc111f790.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   erpnext/accounts/doctype/exchange_rate_revaluation/test_exchange_rate_revaluation.py
	modified:   erpnext/accounts/doctype/payment_entry/payment_entry.js
	modified:   erpnext/accounts/doctype/payment_entry/payment_entry.json
	modified:   erpnext/accounts/doctype/payment_entry/payment_entry.py
	modified:   erpnext/accounts/doctype/payment_entry/test_payment_entry.py
	modified:   erpnext/accounts/doctype/payment_entry_deduction/payment_entry_deduction.py
	modified:   erpnext/accounts/doctype/unreconcile_payment/test_unreconcile_payment.py
	modified:   erpnext/accounts/test/test_utils.py
	new file:   erpnext/patches/v15_0/set_is_exchange_gain_loss_in_payment_entry_deductions.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/accounts/doctype/payment_entry_deduction/payment_entry_deduction.json
	both modified:   erpnext/patches.txt

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally