Q. Does saying Im Yirtzeh Hashem (G-d willing) qualify as saying B’li Neder (no promise made)?

A. The two terms express different ideas. It is common to find in the responsa and rabbinic literature both phrases used together (see Sdei Chemed 8: Yishus, Peas Hasade 12 et. al.)
Meam Loez (Yehoshua 2: 9) quotes that a person dealing with future occurrences should teach himself to say Im Yirtze Hashem, and if it involves the performance of a mitzva he should add b’li neder.
Horav Shlomo Miller Shlit”a added that Hashem certainly is willing that we perform mitzvos, yet we don’t want these to become nedorim.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.