I'd say it's unlikely to have been caused by the manure in such a short time, unless the manure was fresh, hot and un-composted. More likely simple transplant shock. What condition was the rootball when you installed the plant?

Peat mixed into the soil is a good thing, providing acidity and aiding drainage while holding some moisture during dry spells.

Thank you very much for your reply. I got the manure from a garden center. I think it's well rotted one because it has no smell. Are you saying the manure would be fine if I leave the plant there long enough? Or wouldn't it be best to get rid of it and redo the soil with sand and peat?

The root ball was in good condition, I presume. The plant was doing beautifully before planting. I soaked it in water before planting it.

Alternatively, I could mix in a little bit of nutrition (generic compost and/or farm manure) into the soil, balanced by peat. Would this be better?

No...I did not say the manure would be fine if you leave the plant in it. I said it would take more than a week to show negative effects from the manure.

As I said in your other thread, the plant does not need manure, compost or fertilizer. Lean, moist, well-drained soil is its preference.