Leopard1 is a German tank although an old one. So maintaing them is entirly possible for Germany. The idea is probably to trade them for some other tanks, which can be send to Ukraine. The Leopard2 line is at capacity for the next couple years and there is no other tank being activly produced in the EU right now. The only other option is the K2 production in Poland, but that is a South Korean tank design and they do not want their weapons in Ukraine either. Lots of Europeans have asked. The Leclerc production line is still around, but currently closed.