A Pareve knife was only used for cutting bread but was washed together with Fleishig dishes. May I use that knife to cut bread and then use that bread with butter?
If the knife was washed with soap or if the water did not reach, the knife is considered Pareve and you may use the bread for Milchig. However, in the future you should not wash the knife together with the Fleishig if you want it to be Pareve.
If it was not washed with soap, or if it was washed with hot water first before applying soap AND the water reached Yad Soledes Bo, the knife would be considered Fleishig and should not be used to cut bread to be eaten with Milichig if you have another knife.
• When washing with soap it ruins the taste – טעם פגום. (Shulchan Aruch Y.D. 103:1-2).
• See Shemiras Shabbos Kehilchasa Chapter 1 footnote 3 – Yad Soledes Bo starts from 45 Celsius.
• See Shulchan Aruch Yoreh Deah 99:4, the Taz and Pri Megadim there.