The idea of using the terms sourcing and sinking in relation to an analog input module is problematic.
The power source for the 4-20 loop might be located at the field device or at the analog input module.
Regardless of where the power source is, the current leaves the power source, and then enters either the controlling device followed by the precision resistor, or, the precision resistor followed by the controlling device. In either case, the current then returns to the power source.
The power source is "the source" and "the sink"... it's a loop.