Can I use tomato paste in dal?
Tomato paste is far more concentrated, and it's produced by actually cooking down the tomatoes a couple times. Since tomatoes contain a lot of sugar, this will actually caramelize those somewhat and produce a richer, sweeter flavor.  Even canned tomatoes will taste more acidic and less sweet.

So your finished dish will likely follow suit - it'll be a bit sweeter, have a slightly richer flavor, and it will have a bit less moisture.  That's not necessarily a bad thing; you may find that those different flavors interact with the spices in an interesting way.  Just make sure that you don't use a 1-to-1 substitution; because tomato paste is much thicker you'll want to use less of it or its flavor will totally dominate.