Muamar bin Abdullah reported,

He sent his servant boy with a saa of wheat and told him, "Sell this, then buy barley with the price." The boy went and obtained a saa and a bit more in barley. When he returned to Muamar and informed him of this, Muamar said to him, "Why did you do that? Go back and return it. Take only an equal measure for an equal measure, for I used to hear the Messenger of Allah ﷺ say, 'Food for food, equal measure for equal measure.'"He said, "Our food at that time was barley."It was said to him, "But this is not the same as that."He replied, "I fear it may resemble it."

عن معمر بن عبد الله

أنه أرسل غلامه بصاع قمح، فقال: بعه، ثم اشتر به شعيرا فذهب الغلام، فأخذ صاعا وزيادة بعض صاع، فلما جاء معمرا أخبره بذلك، فقال له معمر: لم فعلت ذلك؟ انطلق، فرده، ولا تأخذن إلا مثلا بمثل؛ فإنى كنت أسمع رسول الله ﷺ يقول: الطعام بالطعام مثلا بمثل. قال: وكان طعامنا يومئذ الشعير. قيل له: فإنه ليس بمثله. قال: إني أخاف أن يضارع.