"You can give it back to me tomorrow."
It would sound like Yoda talking.
Actually, your word order can sound more like, "Don't bother if you're going to give it back to me tomorrow." That's because いいです is used as any of, "That's alright," "No, really," "I don't want it," "I'm not interested," etc., depending on context.
When you start the sentence with 返すのは~, you're marking it as the topic, saying, "Regarding returning it, ~".
Okay, so I was confused with this one, because elsewhere I learned that you should say 。。。でもいいです
First of all, they both mean something (that comes before で[も]いいです) is OK. 「Xでいいです」 Secondly, without も it means that it's OK, while other options could be equally OK but they are not mentioned. At least it is so if you compare it to the case with も「Xでもいいです」where も gives it a hint of 'even if', so it becomes something like 'even if I did it this way and not the other' (it's similar to the meaning of も instead of は with the topic I believe)
I infer this from the top answer on the page at the second link (even though I'm not sure since it's in Japanese and I'm still learning)
少しニュアンスが違います。 1つ目は、ペンでも鉛筆でも筆でも、書ける物なら何でもいいという風に聞こえます。 でも2つ目だと、本当は他の物がいい（できればペンじゃない方がいい）という感じがします。
You use のは or のが after a verb (plain form) when you need to turn it into a noun. For example: 読むのは楽しいです=“reading is fun” or “it is fun to read”. You are describing an action using an adjective, so the action needs to become a noun. The の is what turns it into a verb, and the が or は is used because subject (verb-の) needs a particle to show that it is the subject. (I’m not entirely clear about when to use は vs. が; anyone have advice?)
Good explanation about the use of で in this case: http://maggiesensei.com/2014/05/20/%E3%80%9C%E3%81%A7%E3%81%84%E3%81%84-%E3%80%9C%E3%81%A7%E3%82%82%E3%81%84%E3%81%84%E3%80%80vs%E3%80%80%E3%80%9C%E3%81%8C%E3%81%84%E3%81%84-de-ii-demo-ii-vs-ga-ii/
I feel like the sentence is the equivalent to あしたかえしていいです。