Purchase requisition and Purchase Order

hi to all

I raised a PR with GBP currency, when the batch run, SAP converted the PR to PO in AUD!?

This was happened for the first time and now I have issue to do goods receipt and finance can't do invoice receipt.

because the exchange rate is different everyday.

could anyone explain :

1- why this was happened?

2- how we can sort this issue

I do have access to ME22N but the currency is greyed out.

I appreciate your help.

Thanks