Lumps in any shape or form usually are better removed before they grow bigger. Especially in a young hamster as in older animals, it`s usually more risky due to the anaesthetic. This lump possibly was smaller when the hamster was sold to you and not as noticeable and it would be difficult to prove a case against the pet shop if they sold the hamster with or without this issue. You would really need to look at all the facts and proof before you had a case. But without noticing the lump early on, it will be difficult to say the pet shop knew about it before they sold you the hamster.
It`s frustrating, I know, but the hamster will either carry on regardless, or the lump may change shape or grow, so over the weekend, I would check it closely to watch for any changes in size or whether it seems more angry. I would chance surgery if you feel it will give hammy a better quality of life, or you can leave it and if it grows or turns malignant, then the hamster will inevitably deteriorate if the lump was to spread.
At least you have let the shop know what happened. Their response could have been better though than just saying you should have returned him within seven days like a handbag.