Fixes an error that could occur when adding a line to a new subscription. This helps sales teams create subscriptions without being blocked by a currency-related issue.

Impacted version : master
In subscriptions App : 
1. Create a new Subscription
2. "Add a line" --> error : expected singleton of res_currency

#3845 also focuses this bug

This fixes an issue in the rental stock flow where the system used the wrong internal lookup method. It helps prevent errors when handling rental items tied to stock lots, keeping rental operations more reliable.

Manual Forward-Port of #4910