You say something but you don't really mean it: an adjective to describe that offer or a phrase
That type of offer is called an Empty gesture.
Urban Dictionary definition:
to say something without an intention of actually doing it. Making an offer that is not intended to be fulfilled or even taken up by someone.
token, one of whose meanings is, according to the Oxford English Dictionary
- passing into adj. Serving as a token; pro forma; (purely) symbolic; constituting a gesture (only); minimal, nominal, perfunctory....
1962 N.Y. Times Mag. 5 Aug. 11 The current notion that token integration will satisfy his people, says Dr. King, is an illusion.
Example (made up)
She made a token offer to cook tonight, but I knew if I accepted, she would only microwave a TV dinner.
Another possibility is perfunctory, from the OED
a. Of an action, deed, work, etc.: done merely as a matter of duty,
form, or routine, and so without interest, care, or enthusiasm;
carried out with a minimum of effort; formulaic, mechanical;
superficial, trivial.1922 M. A. von Arnim Enchanted April 172 He might answer,—a hurried scribble, showing how much bored he was at doing it, with perfunctory thanks for her letter.
b. Of a person: acting merely by way of duty or routine, or for form's sake and so without interest, care, or enthusiasm; superficial, offhand; formal, formulaic.
Example (made up):
Prof X made a vague, perfunctory offer to write me a letter of recommendation, if I needed one from him, but I decided not to pursue the matter.
Insincere is a great choice here:
"not sincere; not honest in the expression of actual feeling; hypocritical. " http://www.dictionary.com/browse/insincere