The Hebrew verb lavah has a range of meanings: to join or attach oneself to, to lend (in Hiphil), and to borrow (in Qal). The concept of joining or clinging together is its core sense, from which both financial (lending) and relational (joining) meanings derive.
The covenant people of God are called to be joined to Him — the Levites' very name may derive from this root, symbolizing those who are attached to God's service. In Numbers 18:2, Aaron's tribe is to 'join' (lavah) the Levites in service. The financial aspect of lending is governed by the law's concern for the poor: 'You shall lend to many nations, but you shall not borrow' (Deuteronomy 28:12) — a promise of blessing for obedience.