You want to know how the currency of purchase order that is created from Purchase request is determined.
SAP Business ByDesign
Since Purchase request can have many items with different currencies,the Purchase request's header currency gets copied from Purchase request's 1st line item currency. That is whatever currency is maintained for 1st item the same will become Purchase request's header currency.
So when we create a Purchase Order (where only single currency is possible)from Purchase request items, the header currency of Purchase order is copied from header currency of Purchase Request.
Any change to above logic is not supported in the current release of SAP Business ByDesign.
1.If you are a partner: you can help develop this using SDK or please use firstname.lastname@example.org or the Partnerfinder .
2.If you are a customer: if you already have a partner ,ask them to help develop it for you through SDK.
a. Contact your the Account Executive
b. Check the link https://www.sap.com/uk/services/advisory-development/application-development.html#contact-us
3. Alternatively you may check the Customer Influence Forum to submit an idea: https://influence.sap.com/sap/ino/#campaign/886
KBA 2912560 - How the currency is derived in manual Purchase request
Purchase order currency, currency wrong, PO currency wrong,determination of Purchase order currency , KBA , AP-POP-PO , Purchase Order , AP-PRP-PR , Purchase Request , Problem