The general solution to exit a loop when a condition is met, is to incorporate a break statement in whatever conditional decides the loop has achieved its purpose early.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/break

I've made a simple snippet that runs a million cycle for-next loop but terminates after 10 loops when a condition is met, using break.

let loopNumber = 0;

for (let i=0; i<1000000; i++)
  {
    loopNumber++;
    
    if (loopNumber==10) {
    break;
    }
    
  } // next i;
  
  console.log(loopNumber);

Not sure I understood completely what you actually want but lets try:

   async function fulfillReward(reward) {
      await fetch(
        `https://api.twitch.tv/helix/channel_points/custom_rewards/redemptions?broadcaster_id=58606718&reward_id=08d5e2d9-ddd7-4082-bc78-39b06b35cd68&id=${reward.id}`,
        {
          method: 'PATCH',
          headers: {
            'client-Id': process.env.TWITCHBOT_CLIENT_ID,
            Authorization: `Bearer ${process.env.TWITCHBOT_ACCESS_TOKEN}`,
            'Content-Type': 'application/json',
          },
          body: JSON.stringify({
            status: 'FULFILLED',
          }),
        }
      )     
    }

If you just receive the reward as a parameter you don't need to declare functions inside the for (please don't ever do that)

Then you call it sending the reward parameter

 if (currentRewardUsername === input.toLowerCase()) {
        countDocuments(discordID)
          .then(() => {
            return findOneUser(discordID)
          })
          .then(() => {
            fulfillReward(currentReward)
            interaction.reply('success')
          })
          .catch((err) => console.log(err))
      } else if (currentRewardUsername != input.toLowerCase()) {
        return interaction.reply(`The Twitch user **${input}** has not redeemed the channel reward!`)
      }

I updated your code here https://pastebin.com/clone/7k5WNhmD, but cant really test it, so please let me know if you need something else

try this.

for (let i = 0; i < rewards.length; i++) {
      async function fulfillReward() {
        await fetch(
          `https://api.twitch.tv/helix/channel_points/custom_rewards/redemptions?broadcaster_id=58606718&reward_id=08d5e2d9-ddd7-4082-bc78-39b06b35cd68&id=${rewards[i].id}`,
          {
            method: 'PATCH',
            headers: {
              'client-Id': process.env.TWITCHBOT_CLIENT_ID,
              Authorization: `Bearer ${process.env.TWITCHBOT_ACCESS_TOKEN}`,
              'Content-Type': 'application/json',
            },
            body: JSON.stringify({
              status: 'FULFILLED',
            }),
          }
        )
      }

      const currentReward = rewards[i]
      const currentRewardUsername = currentReward.user_name.toLowerCase()
      

      if (currentRewardUsername === input.toLowerCase()) {
         function doSomething() {
              return new Promise((resolve, reject) => {
                 countDocuments(discordID)
                  .then(() => {
                    return findOneUser(discordID)
                  })
                  .then(() => {
                    // All (x) amount of rewards get fulfilled instead of the first matching result 
                    fulfillReward()
                    interaction.reply('success')
                    resolve(true);
                  })
                  .catch((err) => console.log(err))
            })
         }
          
         const ret = await doSomething();
 
         if (ret)
            return ;
            
      } else if (currentRewardUsername != input.toLowerCase()) {
        return interaction.reply(`The Twitch user **${input}** has not redeemed the channel reward!`)
      }
    }