The absolute minimum balance for every active Stellar account is 1 XLM, therefore when making a payment you should make sure to leave at least 1 lumen in your account.
Minimum balances help protect the network from spam account creation. Following the Stellar Network protocol, each new held asset (trustline), signer or active trade offer requires an additional 0.5 XLM reserve.
For example, an account with 1 trustline and 2 offers would have a minimum balance of 2.5 lumens:
1 XLM account minimum balance + (0.5 XLM base reserve * 3 entries) = 2.5 total XLM reserve.
- Data entries
With reference to the Stellar Network protocol, when creating an offer for sale, the funds involved in the offer are postponed to the reserved balance and cannot be used until the order is canceled or executed.