Is signing blank promissory notes in exchange for a design service that has no fixed price considered normal, dubious, or unlawful (haram), and what should I do in this situation?

It is never permissible for the borrower to sign blank promissory notes. However, there is no harm in it if suspicion is absent and the transaction is with a trustworthy Muslim, especially if he is a partner, as his hand is one of trust. If doubt or suspicion arises without clear evidence, then there is no objection to not signing. But if matters reach the point where there is clear evidence of injustice and the loss of a person's right, then it is not permissible to sign a blank note, and one must repel injustice and support the oppressed.