Suppose I have a (closed) circuit with a voltage source $V$, a resistance $R$ and a current source $I$ (in this order). Is it true that the voltage across the current source is $V-RI$ ?
Your source voltage is a pressure of electrical charge, with a degree of resistance (obstruction) somewhere in-between zero and infinity, towards a lesser pressure of electrical charge. If the source pressure is not kept constant, by electrical input to the source, then this source pressure will diminish for as long as your current is kept constant. So any pressure released into the circuit will be maintained magnetically in the circuit itself, voltage loss being released in far greater maginitude at your current source. In a given span of time, voltage loss is far greater than the magnetic force because I (current) is measured--in time. Constant Voltage / R = I. Since V = IR, V across the supply circuit itself will equal the total pressure in the closed circuit minus the magnetic force between the voltage source and the point of supply. So in reality, the voltage (electrical pressure) in your current source is maintained only in the magnetic force, which is in addition to your constant source voltage. If a finger (God forbid) or something shorts the circuit, the source voltage will be imposed on, but the magnetic flux in the circuit is maintained. The bases I'm using are books studied over time, one in particular titled simply, "Electricity," whose subtitle and author's name elude me at the moment, and some others including an elementary college physics course.