Inverting regulators and sinking current
Consider the topology of an inverting buck converter:







simulate this circuit &ndash; Schematic created using CircuitLab

The regulator output does not sink current. The coil does!

When the MOSFET inside the regulator is conducting, there's a current running clockwise in the left loop. Then the regulator opens the MOSFET, but the current through the coil cannot be turned off in an instant. That's why there's now a counterclockwise current in the right loop.