でよければ is an expression that is constructed with the 格助詞(かくじょし) (case marking particle) で, and the potential form of the
い-Adjective いい, よければ. The literal meaning of ‘if with (A) is good/fine’, or ‘if by (A) is good/fine’ is quite similar to the standard translation, which is ‘if (A) is ok’.
でよければ will always follow nouns or noun-phrases.
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焼(や)き肉(にく)でよければ、うちの近(ちか)くに美味(おい)しいところがあるから、そこに行(い)かない?
If yakiniku is fine, there is a good yakiniku place near my house, so do you want to go there?
中古(ちゅうこ)で良(よ)ければいいのが沢山(たくさん)ありますよ。
If used ones are fine, we have many good options.
でよければ is a phrase that is often used in relation to oneself, in order to politely offer some sort of help, advice, or service.
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私(わたし)でよければ手伝(てつだう)うよ。
If you don't mind, I can help you. (If I am a suitable option, I can help you)
先生(せんせい)でよければ、先生(せんせい)が写真(しゃしん)を取(と)ってあげるよ。
If you don't mind, I can take a picture for you.
As
で is being used in its standard meaning of ‘with’ or ‘by’,
でよければ implies that the speaker is enquiring whether the quality/ability of (A) is good enough to achieve some specific result.