The Stammkapital was required for the UG, and it's very much locked since the whole purpose of that is to take liability.

So for a UG it's a Kommanditeinlage. But it doesn't change the fact that you can spend this money for business purposes. You just can't wire it back to yourself or pay it to yourself as a wage. Feel free to buy office furniture or company devices with it.

It's a common misunderstanding that this money needs to be reserved somewhere for liability purposes. That's not correct. You can spend it, both in a GmbH as well as in a UG. Look it up youself (e.g. https://www.anwalt.de/rechtstipps/stammkapital-muss-ich-das-...)