It is perhaps easier to consider a battery as the voltage source with no internal resistance.
Within the battery a chemical reaction maintains a constant potential difference $V_b$ across the terminals of the voltage source.
Now connect another voltage source $V_s$ and a resistor $R$ across the terminals of the battery with the negative terminals of the battery and the voltage source connected together then a current $I_b$ flows in the circuit.
Applying Kirchhoff's voltage law gives
$V_b - V_s = I_b R$
If $V_b > V_s$ then the current $I_b$ flows out of the positive terminal of the battery.
Changing the value of the resistance of the resistor does not change the potential difference across the terminals of the battery.
If $V_b<V_s$ then current flows into the positive terminal of the battery but the voltage across the terminals does not change even when the resistance $R$ and hence the current $I$ changes.
You can interpret this as the battery being recharged with electrical energy being pumped into the battery and reversing the chemical process within the battery.
So electrical energy is flowing into the battery.
Now a similar analysis for a current source yields the fact that the potential difference across a current source has to follow the potential difference imposed on it by an external circuit $V_b = V_s$ whilst still maintaining a constant current.
The interpretation of this effect is a little more difficult to understand but ultimately means that when the terminal of the current source out of which the current flows is at a negative potential relative to the other terminal of the current source (the positive and negative signs in the diagram are reversed) electrical energy is flowing into the current source and perhaps yet again is recharging a battery which is part of the current source circuit?