Revaluation of a purchase order price due to a material price change

We request to tally a revaluation of a acquisition bid owed to a worldly terms change. This works successful the institution currency but does not enactment successful a functional currency antithetic to the institution currency, utilizing the MRNB transaction ( yes it exists successful S/4HANA Cloud Public edition!)

The worldly currency is the institution currency. The PO is successful the functional currency (which is antithetic to the institution currency).

The concern script is that the magnitude we wage for the merchandise (sucrose from sweetener cane) changes each 4th and we request to backmost wage the suppliers for sucrose supplied anterior to the alteration successful price. 

Any ideas, enactment aorunds oregon suggestions arsenic to different method of achieving this result?

SAP S/4HANA Cloud Extensibility 

SAP S/4HANA 

SAP S/4HANA Cloud Public Edition