Changing currency of an offer

So I’m trying to change currency of the offer basing on the dropdown ‘currency’ value. So I have an API data about currencies values which is for example ‘USDvalue’ in my data base and also data about this particular offer’s value in dollars (and the currency in which this offer is listed, if it’s not dollars) that is created when the offer is listed by the user, that’s for example ‘OfferUSDv’.

When the ‘dropdown currency’ value changes (let’s say it’s for example PLN) there’s a workflow that calculates 'this offer’s OfferUSDv * PLNvalue (everything is data type ‘offer’) but somehow I can see that in debugger it equals 0. And text ‘offer price’ shows only ‘PLN’ - text I typed myself. Also, I just started testing this so only one offer has ‘OfferUSDv’ based on which it should calculate this offer’s value in other currencies. Here’s a few screenshots of this setup, let me know what’s wrong and why this value equals ‘0’ and if it’s 0 why ‘text prce’ doesn’t displays ‘0 PLN’?

Okay I the problem is that the currency value is offers data type and I can’t do ‘…*This Offer’s CURRENCYvalue’ since most offers don’t have a currency value and no workflow gives it to them. So there’s only one question; as what should I save the currency data that I got from external API so I can make a condition just like on the picture? (I don’t want to save it as a current users thing because that would probably cause too many API calls)