Is quoting and making slight changes to the writings of others to produce da‘wah materials, without citing the source, considered theft and does it invalidate the earnings acquired from it?

There is no harm in benefiting from the beautiful phrases of others and their creative presentation methods, as long as it is not a theft of entire articles or complete sections. The writer should not deceive others into believing that what he has written is his own creation. Rather, he should indicate that his effort was in collecting, coordinating, and selecting, for the Prophet, peace and blessings be upon him, said: "The one who pretends to have been given what he has not been given is like one who wears two garments of falsehood."