Yes, the collection agency (or a creditor) can refuse a payment arrangment for being "too small." They are entitled to all of their money immediately, if it is a valid debt; if they choose to accept a payment plan, that is voluntary on their part; being voluntary, they have complete discretion as to whether or not to consider any proposed payment offer as reasonable or acceptable, and can refuse to accept one they consider unacceptable. The law does not make your economic situationa  defense, and does not require the collections agency or creditor to take account of what you can afford to pay.