automatically change the posting invoice date
Default posting date set to Work Date would solve this if the invoices are created and posted within the same day.
If it is on different days, then You can use Batch posting of Purchase Invoices.
For example, if You change status to Released only for invoices that are ready to be posted, You can use this as a filter, place the checkmark to Replace Posting Date and VAT Date, and enter the new Posting date to be used on all invoices that will be posted.
But be aware of the warning message:

So better to try it out in a Sandbox environment or test company, to see how the currency rates are updated.