I have seen some topics related to this issue, therefore I would like to clarify the standard behavior in SRM about currency that is adopted in PO when this one is created from Shopping Cart.
Currency in Shopping Cart:
When Shopping Cart is created, item can be added in any currency. However, total value in Shopping Cart will be displayed in currency defined in PPOMA_BBP for Shopping Cart creator – “Local Currency” attribute.
For more details, you can see following SRM Wiki link:
Currency in Purchase Order:
In order to have a Purchase Order created and ordered, a Supplier is mandatory.
Supplier can be assigned in Shopping Cart directly or in Sourcing Cockpit, then a Purchase Order will be created as follow on document.
Currency adopted in this Purchase Order will be the currency maintained in Vendor Master for a specific Purchasing Organization (Purchasing Organization assigned to the user who created the Shopping Cart).
This information can be seen in table BBPM_BUT_FRG0061. Supplier GUID should be provided as input parameter, then it will be displayed the Purchasing Organization that this Supplier is assigned, the currency assigned to this Vendor and the output medium defined.
When PO is created directly (without Shopping Cart), currency will be taken directly from Vendor Master.