Does the saying "There is no denial in matters of ijtihad" refer to absolute denial or to denial restricted to denunciation and overstepping boundaries, and does it apply to absolute ijtihad or to that which does not contradict an agreed-upon fundamental principle, and who said it?

The denial negated in the principle "There is no denial in matters of ijtihad (definition)" refers to the denial of explicit prohibitions firmly established [in Islamic law] through changing them by hand, or overturning a judgment, or preventing a fatwa (definition), or denouncing. As for scholarly discussion and advice, these are not included in that. Scholars have agreed upon this principle and have adduced religious texts as evidence for it.