The OPīs post is unreadable.

As near as I can tell, the OP wanted to travel to Germany at a time when only German residents could enter the country due to COVID restrictions. The airline denied boarding, probably for good reason. It has nothing to do with screwing passengers to get more money, but to avoid being fined by the arrival country for allowing ineligible passengers on board.

As mentioned, a potential solution could have been to quickly buy a one-way refundable ticket from either Madrid or Germany to London. Although perhaps at the time that would have involved a hotel quarantine package. But it does seem like the OP was in no particular hurry to get to Germany, rent contract or not...

In any event, the only way for this thread to be helpful would be for the OP to try again, this time avoiding the stream-of-consciousness rant and sticking to the facts.