I got sent funds but cannot get them
It is difficult to cancel a Bitcoin transaction after it's been sent, and it cannot be done by the recipient, only by the sender.  If the transaction has been confirmed, cancellation becomes impossible.

However, the funds are not lost.  They will be waiting in the address until you claim them.  You can either eventually finish installing and syncing Bitcoin Core (possibly on another computer), or you can import your wallet.dat file into some other software with lower resource requirements, and you will be able to spend your funds then.