the only way to make this work is to treat the outsourcing partner as part of your team. this means you have to develop full trust in the beginning of your working relationship. this trust will be the basis of your working together. this is the most risky part of such a relationship.
initially you can start with giving them a smaller task, which doesn't require too much internal knowledge. as you see how they progress, and how the two teams (yours and them) can productively work together, you can increase your level of commitment to them, as will they do the same. in a manner of months, you can ramp up the amount of work you give them, and they will allocate more people of their team to you. if it doesn't work out, you just cut the relationship and look for someone else.
as trust builds up, you don't have issues anymore of sharing your assets with the outsourcing team. you also don't have accountability issues, like double-checking their timesheets, etc. this also means that efficiency increases. trust really saves money for everyone.
in an ideal setup, you'll end up with a long-term partner, that is at least as good as what you could get in-house. they will also appreciate this long-term and stable relationship. the benefit will be there for both: you'll get a capable team that is cheaper than what you'd have in-house, and is at least as good. you'll also have more flexibility in the amount of load you place on them. you also don't have to worry about recruitment, training, etc. they will love the long-term and stable customer they are working for. they will enjoy the challenges you give them. and most probably they will make more money than what they could make with a local client.
also note that outsourcing is not always about price, although price is always part of the equation. but its useless to go just for price - as you pretty much get what you pay for. but, you can also aim for an external development partner that is high quality, and not 'dirt cheap', but indeed, cheaper than a comparable high quality in-house team, or a team where you live.
(disclaimer: I run such an outsourcing company in East Europe, and we work for US customers on a long-term, high-quality basis)