I understand and we do make exceptions sometimes in such cases. But, the problem is not with whether you would bring a good part or not, to me. If you brought your own part I could not warrantee it and thus I could not warrantee the job and I don't do it any other way. I do not get involved with "I warrantee the labor" "you warrantee the part". This always leads to disagreements. I have no disagreements with my warrantee policy. I warrantee everything. No discussions.
I am pointing this out to the audience more than you personally. The economic reasons for not installing customer supplied parts are much to lengthy for this post.
Bosch Master, ASE Master, L1
33 years MB technician