Sure, another developer could come in and take over (and the acquiring company will probably want to get their own developers on it if they have an in-house team), but finding that trustworthy developer and getting them up to speed will be non-trivial for them without you.
Being open and transparent has served me well so far in the professional world. Tell them you made a mistake and ended up doing the original project far below your usual rate and that you're now currently busy with other projects. Let them know how long the original project took so they have expectations on how long new modifications might take.
Tell them that although you like working with them, in order for it to be worth your while and to turn down other opportunities, you'd need it to be in line with your usual rate of 80€/h.
You'll also probably want to get them to agree not to let the acquiring company know what your hourly rate is, as if they are enterprise-y you can definitely increase your rates (which is also warranted, since you'll be dealing with a lot more bureaucracy).