Innovative.

I like the concept of crossover of fund-raising platform and freelance marketplace. It allows for organization of exchange that is smarter from societal design / functional viewpoint (which is kind of things I like a lot).

I can't give advice about how to get users, but will discuss the concept itself. Remember that you might miss better ideas if you guide your project solely by users' reactions.

Note that if your service doesn't require accountability from creators for spending earned money, it will functionally be simply a Taskrabbit clone with a light decoration of "fundraising causes" added and a focus on local community. This seems relatively hard to justify when there is Taskrabbit already.

I think that most people care little whether someone they hire needs to "pay rent" or anything else, unless the person is significant to them so that they would help them without strict conditions / obligations. So, your idea works in the case of friends or other significant persons to one, without need to make any kind of guarantees about the spending of payments. Allowing only this and limiting causes to "personal", though, would be an unfortunate omission in the design. Consider another way to use your service: creator in their campaign sets promises about spending of payments which legally (or otherwise effectively) bind them, and then unrelated strangers who nevertheless like these promises, find and accept the offer. So your service would have both angles of helping people from local community personally and helping specific projects from anywhere.

Political meaning of this: In essence, the obligations become another half of the goods provided, which may rise the amount of payment. This will result in that offers of products unrelated to each other "purely utilitarianly", will get "married to each other", become interdependent in bundles more fit than independent offers, which is sign of market participants' stronger moral control of society.

My idea of how your service should handle it: a creator creates a set of service offers and a set of causes independently, then links each offer to a subset of causes (in simplest cases, there will be only one cause or all of creator's offers will be linked to all their causes). In each offer, a creator sets one or more default proportions. A buyer either selects a default proportion or sets a custom proportion; it defines which fraction of payment goes to which cause. Every transaction records the proportion, and the obligation-checking mechanism would calculate from all transactions target sums to use in checking. There could be a dummy cause which is checked trivially so that a creator can consume a pre-agreed fraction of payment freely. Example: 70% - Some Noble Cause (creator is accountable for it in a concrete way); 30% - dummy cause (promise is trivially fulfilled, e. g. creator is free to spend this any way). Every element of proportion could have a part fixed by creator and a part variable by buyer, so you could support complex rules for proportions acceptable by creator: for example, 30% on Cause A, 10% minimum on Cause X and on Cause Y, 20% more are distributed between causes X and Y by buyer's choice and remaining 30% are free spending. You can see here how this can become a lifestyle: people would pledge fraction(s) of their earning to charities or generally to causes, they would share their campaign pages... this could become a social media! Consider this. Including the dummy cause, every proportion sums to 100% minus fees for your service. You can also add a kind of payment-doubling mechanism: a buyer allocates X% of payment to a cause, yet Y% is verified spending of, where (Y-X)% automatically comes from somewhere else.

All in all, I think this supplemented variant of the idea is very good and interesting and I want to see it in reality. Thank you for developing this kind of ideas.