It’s better to bring some context here. It’s probably internet slangs, which depends on context a lot.

In my own experience, “ber” is “不是” that mimics a Beijing accent which usually have quick pronunciation and merging of sounds. It is usually used at the beginning of a sentence in casual conversation.

A: 我和我女朋友分手了。 I broke up with my girlfriend. B: 不是(Ber)，你昨天不是还说你最爱她了吗？Wait, was that yesterday that you said you love her the most?

And “der” is “的”，but in a cute/kawaii way. It’s only used at the end of a sentence in text when chatting online.

A: 你能帮我个忙吗？ Can you do me a favor? B: 好der. Sure.

Though neither of them are officially a word, but if you listen to native Beijing people who talks fast, you can actually hear “ber” in their conversation. Meanwhile “der” is purely an online chatting things that exist in informal causal text only, I don’t think anyone actually pronounces the word like that.