(a) The RASHBAM (Shemos 11:2, 12:35,36) explains the word to mean not that they lent, but that they gave their belongings to the Jews as a full-fledged gift; that is, "they gave in to their request." This use of the word also appears in Tehilim 2:8, and it means that their request was fulfilled.

(b) Some explain it simply, that the Jews asked to borrow the belongings of the Egyptians. As to your question, how does one borrow or lend when there is no intention of it being returned, that is exactly the point. The Jews deluded the Egyptians into thinking that they would return their belongings. They did it in this manner because the Egyptians owed them 210 years worth of payment for their work (as the Gemara says in Sanhedrin 91a, that this was the decisive claim that caused the court of Alexander the Great to permit the Jews to keep the money and not have to return it to the Egyptians). It is similar to stealing back an object from a thief which he stole from you to begin with. Although the Gemara prohibits such a measure normally (Berachos 5b), that is in a case where the object is stolen back from the theif. If the theif hands the object to you willingly (even as a loan), it may be permitted to withold it.

Some add that Hash-m judged the Egyptians measure for measure: they tricked the Jewish people with words by saying that they just wanted the Jews to do light work for them. The verse says, "With a guileful person, act guilefully." The Egyptians were punished measure for measure by being tricked with words, when the Jews said that they were just going to borrow their belongings and ended up keeping them.

-Mordecai