Most electric motors are constant torque devices (until they reach rated speed, at which point they become a constant power device), so the amount of torque they produce is consistent over their speed range.

Current is directly related to the torque (load) applied, and speed is directly related to voltage. (this also applies to AC motors/VFDs, but you have the added complication of frequency controlling speed, but the basic current/torque and voltage/speed rules still apply).

That is the theory behind motors, but the actual speed/torque curves will not be perfect straight lines due to other factors, but they'll be near enough for basic calculations.

A good motor speed controller will always limit current to the motors rated current. You can try driving more current through by increasing voltage at a given speed, but that is when you're likely to damage the motor, as you're essentially pushing more current through the motor than it's designed for, which will likely lead to saturation (aka the motor magnets aren't physically capable of producing any more torque), and overheating.

This means that the OP's 900W (90V x 10A) motor with the maximum spindle speed of 2000rpm, gives a constant torque of 4.3Nm at the spindle.

At 100RPM it should still produce 4.3Nm, but as it's only doing 5% of it's rated speed, it will only be capable of producing 45W of power.

To put it context, you can probably get more than 4.3Nm turning a typical screwdriver handle.