"I'm honored that you would ask me to help, but I'm unable to add anything else to my workload right now."
"I can't lower my price, and I hope that you find someone within your budget."
"I know I'm working from home, but I'm still maintaining my normal business hours."
"I can't offer my services for free."
"I need help with my workload because I can't manage everything on my plate."
"I'm not able to chat right now; I need to stay focused on my work assignment."
"I'd rather talk by phone instead of emailing about this topic."
"I don't feel comfortable talking about politics at work."