traveling before getting residencia card

hi all! i'm new to the forum, and have been reading through various posts in an attempt to find an answer to my question, but haven't come upon anything. i'm an american citizen living in madrid with a residencia y trabajo visa. i entered spain with my visa at the end of september, so it is valid until the end of december, ie 90 days. however, i heard that it is really only valid for 30 days after it was stamped because in that time i have to get my residencia card. i've already gotten fingerprinted for it, and now i'm just waiting the 40 days to pick it up. however, i'm travelling to london this weekend, and i'm wondering if i'll have problems getting back into spain. i have my residencia receipt saying that i can pick it up 40 days from the date on the receipt, but i'm not sure that's a valid document:) also, i know traveling within europe is usually fine and they don't check american visas/passports, but i'm also going to argentina at the end of the month, and am wondering if entering spain will be a problem. please let me know if you have any information or experience with this kind of stuff! thanks in advance!