Giving cards away in Catan
Permitted:

One can trade a number of cards for a different number of cards.⇒ You could accept a sheep.

Forbidden:

One can't trade for nothing.⇒ You can't accept nothing.
One can't trade for resources of the same type as those being traded away.⇒ You can't accept any amount of brick or lumber.

The following are the rules concerning Domestic Trades:

On your turn, you may trade resources with the other players (after rolling for resource production). You and the other players negotiate the terms of your trades—such as which cards will be exchanged. You may trade as many times as you can, using single or multiple cards, but you may not give away cards (i.e., “trade” 0 cards for 1 or more cards).
Important: While it is your turn you must be a part of all trades, the other players may not trade amongst themselves.
Example: It is Pete’s turn. He needs one brick to build a road. He has 2 lumber and 3 ore. Pete asks aloud, “Who will give me 1 brick for 1 ore?” Beth answers, “If you give me 3 ore, I’ll give you a brick.” Cooper interjects, “I’ll give you 1 brick if you give me 1 lumber and 1 ore.” Pete accepts Cooper's offer and trades a lumber and an ore for a brick. Note Beth may not trade with Cooper, since it is Pete’s turn.

The following are the rules concerning Trades:

After you roll for resource production, you may trade with other players (domestic trade) or with the bank (maritime trade). If you decide not to trade during your turn, no one can trade. You may trade with another player between your turns, but only if it is his turn and he elects to trade with you. You cannot trade with the bank during another player’s turn. You may not give away cards. You may trade as long as you have Resource Cards. You may not trade Development Cards. You may not trade like resources (e.g., 2 wool for 1 wool).

Emphasis in both quotes is mine.