How can I avoid stamp duty when buying a property with my daughter but maintain some control over it?

If we gift our daughter money to help her buy a house is there any legal agreement that can be drawn up that means we can still maintain a 50% share of it. We are concerned re gifting in case at a point in the future we fall out for any reason but we would like to avoid stamp duty as she is a first time buyer. This feels like the easiest if it can be done with a legal agreement.

We would put in 50% of the value and my daughter the remainder, partly in cash and partly with a mortgage.

We have heard of trusts but are not sure if these can only be used when buying outright in cash? Or indeed if they are suitable for our situation.