I recently needed to set currency for a record in CRM in my plugin. It sounded a good piece of code to refer to my blog as it is often needed. Here it goes:

EntityReference currencyType = new EntityReference();
currencyType.Id = “(The Guid Of The Currency Type Goes Here)”;
currencyType.LogicalName = “transactioncurrency”;

entity.Attributes.Add(“transactioncurrencyid”,currencyType);

For JS reference refer my older blog post: Click here

Hope it helps!