How do I record Advance foreign remittance against Proforma invoice with out enable of Exchange rate / Multi currency option

What is the local currency of your customer? Are you using INR as the home currency? You may have a workaround without enabling the multicurrency feature. 

Thanks for reaching out to us with your inquiry, abhirami-manokar.

Generally, you will have to turn on the Multicurrency feature for this process. With your current situation, you will have to manually calculate the exchange rate from the foreign to your home currency.

Then, record the remittance against the proforma invoice as you normally do. 

I'm sharing this article in case you decide to enable the Multicurrency in the future: Turn on and use multicurrency

Please let me know if there's anything else you need. I'll be around to help you out again. Take care and stay safe!